ddb@mrvax.DEC (DAVID DYER-BENNET MRO1-2/L14 DTN 231-4076) (07/20/84)
Alternative suggestion for hack densitometry (I'm the guy who originally raised the issue, I've been playing with it and reading the various suggestions provided privately and on the net): Use your enlarger for the light source. The enlarger is designed to provide even illumination over something the size of a negative, has a negative carrier which safely holds the negative, etc. I have the enlarging attachment for my Luna Pro F, which lets me make readings on the baseboard conveniently. Brian Diehm ain't kidding when he stresses the need for careful setup. Make sure all lights in the room are off (other than the enlarger lamp; picky, picky, picky). Make measurements at a standard position on the baseboard (because the illumination won't be even, and the lens will have some falloff towards the edges). Be sure the lens on the enlarger is set to the same place each time. Be sure the height of the enlarger head hasn't been changed. Be sure the focusing belows are at the same extension. Be sure the bulb in the enlarger hasn't changed its output significantly since any previous readings you wish to compare to. Be sure the power is at the proper voltage. Since many of these things can't be controlled accurately enough, you can't really make density readings; however, you can COMPARE the densities of two negatives, if you make the readings one after the other, because then you really can be sure most of those variables haven't changed, and you reduce the probability of significant variation in the bulb condition and power voltage to the point where I don't worry about it. For zone system calibration work you want to read density above base + fog anyway; so I keep a developed, unexposed, negative for the film I'm working with around. I read this first, then measure the difference between this and the negative I want a reading of. Now for the complaints. I'm not convinced I'm getting 1/3 stop accuracy with my trials at this. My meter (and every other meter I've played with) seems to have "flat spots" or something on the adjustment wheel. There are places where 1/2 stop wheel turning doesn't make much difference in needle position (this is a Gossen null-reading meter; you adjust the dial until a meter needle points to zero). Also, since what I want to measure is third stops or a little better, I'd like to feel I was accurate to say half that. I've tried interpolating on the scale, and I'm convinced I was guessing more than half the time; not good enough. On the bright side, though, this cost me only $35 (for the enlarging attachment to an existing meter), and I could have done it for less if I'd had anything I was willing to consider using as my standard, even, light source that didn't shine DOWN. I'm certainly not prepared to go out and spend $1300 for an Eseco/Speedmaster densitometer accurate to .01 density.